1. Field of the Invention
The invention relates to solar control coatings on glass and, in particular, to double glazing units incorporating highly reflecting solar control coatings of silver metal.
The term `solar control` is used to refer to coatings used on glass to reduce the amount of solar heat which enters buildings through the windows in order to avoid overheating within the building and/or reduce the load on air-conditioning plant cooling the building. A wide range of solar control coatings of different composition and thickness, to give a range of colours and performance, is commercially available.
2. Description of the Prior Art
Typically, solar control coatings function by reflecting a proportion of solar heat incident upon them. They also cause the coated glass to absorb an increased proportion of the incident solar heat, and much of that absorbed solar heat is re-radiated towards the outside. Even glasses with coatings which are generally considered as highly reflecting, for example REFLECTAFLOAT (trade mark) glass from Pilkington Glass Limited, depend heavily for their performance on the absorption of solar heat. As a result of such solar heat absorption, the coated glasses become hot in use and it is frequently necessary to toughen the coated glass in order to avoid breakage through overheating. It would be desirable to have a coated solar control glass which could be widely used without the need for toughening. This is particularly important for coated glasses which are not toughenable i.e. coated glasses which lose their useful properties on toughening. To produce such glasses in coated form, it is necessary to toughen the glass and then coat it. This is an expensive procedure, since in practice it involves coating relatively small pieces of toughened glass, rather than large stock sizes which can be cut up for use later.
Moreover, as indicated above, different coatings are required depending on the appearance and properties required in the product. It would be desirable to have a coated solar control glass which could be used to provide glazings with a range of reflection colours.
British patent specification 1,152,691 describes the use of heat reflecting metal or metal oxide coatings to protect body coloured heat absorbing glasses from overheating, with the consequent risk of fracture, due to solar heating. To protect the heat absorbing glass, the coating is disposed between the glass and the sun. In one example, a pane of heat absorbing grey glass is used as the inner pane of a double glazing unit, with a gold coating on the surface of the pane facing the airspace of the unit. The outer pane may be of clear float or of `slightly coloured` glass.
British patent specification 2,174,412A describes a double glazing unit incorporating two coatings. One of the coatings is a low emissivity coating comprising doped tin oxide and/or doped indium oxide, and the other coating is a heat reflecting metal oxide coating comprising at least 30% tin and at least 30% titanium calculated as weight percent of the respective dioxide in the coating. The coatings may be on the same or different panes, and one of the panes may be of body coloured glass. Example 3 describes a double glazing unit having an outer pane of bronze float glass with the heat reflecting metal oxide coating on its outer face (i.e. away from the airspace), and an inner pane with the low emissivity coating on its outer face (i.e. towards the air space).
U.S. Pat. No. 4,069,630 describes a double glazing unit having a low emissivity tin oxide containing coating on the inner face (i.e. towards the airspace) of the outer pane. To mask the iridescent colour of the coating, the outer pane is of coloured heat absorbing glass.